[Libreoffice-bugs] [Bug 139296] scrolling document canvas down & up can cause a line of text to "tear" with loss of registration, it clears when the line moves out of view far enough or view passes to a new page

[bugzilla-daemon at bugs.documentfoundation.org]

https://bugs.documentfoundation.org/show_bug.cgi?id=139296

--- Comment #52 from spokanemagneto at gmail.com ---
This may be it! I know I’ve said this before, but the problem with
troubleshooting a system as large and complex as LibreOffice Writer is that it
has so many interconnected features (some necessary, some just bells and
whistles) that it makes seeing the forest for the trees extremely difficult,
especially for a newcomer to the product like myself. 

I now suspect that the cause of the text tearing issue is two-fold, one
relating to a feature of the product I did not like at all upon first using the
product—that being the sidebar, and the other regarding a feature that I wrote
a bug report on earlier—that being the fact that the so‑called Smooth Scroll
feature works exactly opposite of what the name implies (which is to say that
by selecting that checkbox the text scrolls in a somewhat jerky manner and
deselecting it makes for somewhat smoother scrolling). 

How I came to this conclusion. A replier to this thread attached a read-only
document that did not exhibit the text tearing issue. Another replier, or
perhaps the same one, mentioned earlier that he wrote either the help or user
guides for LibreOffice and verified that the text tearing issue happened all
the time and noticed that the problem only seemed to occur after editing a
document. 

So, what I did last night was first spend an hour playing with one long,
heavily formatted document that I could get to exhibit the text tearing problem
quite frequently by using the up/down vertical scroll bars buttons. I then
selected the so-called Smooth Scroll option, and that combination made the
document exhibit the text tearing issue 100% of the time (you might have to
scroll, stop, scroll some more before noticing the problem, but it was 100%
repeatable at some point scrolling through the document). It did not make any
difference if I did any editing or not, whether I had the sidebar displayed or
not (although I had it open and was using it earlier), or what the zoom factor
was (as long as it was over 100% (see earlier comment about Optimal view). 

I then hid the sidebar, closed the document, and finally closed Writer. I then
immediately re‑opened Writer and the document I had been using. I spent the
next two hours scrolling every which way (scroll bar up/down arrows, scroll bar
handle, mouse wheel, and touch pad) and never once had a repeat of the text
tearing—not even minimal tearing. 

This morning I spent another couple hours to verify my findings. As long as I
never open the sidebar and did not select the so-called Smooth Scroll feature,
no problem. However, if I did turn on the Smooth Scroll, I noticed that
although I never observed any perceptible text tearing, what I did notice was
that when I clicked on the Toggle Automatic Spell Checking that there would, on
occasion, be a noticeable redrawing of the page, albeit ever so slight
(changing the line spacing and/or text size just enough to see an overall shift
in the line placements on the page, whereas with the Smooth Scroll option
deselected, this does not seem to ever happen (the display is rock solid),
although that could simply be due to it being an extremely rare occurrence). I
did, however, notice some text tearing (on very rare occasions) on the line
immediately above the highlighted word I was going to edit, but considering how
much worse and repeatable the text tearing issue became, especially in
combination with using the sidebar, I think it is definitely worth pursuing. 

So, the culprit seems to be two-fold, with the factors being the sidebar with
an assist from the Smooth Scroll feature (I still have no idea why anyone would
ever want the scrolling not to be smooth, but I am grateful in this case that I
can turn it off to get the product to work as expected…or at least to approach
that goal). So, this is about as far as I can go with this. I've spent
countless hours narrowing this down, complicated by the steep learning curve of
getting familiar with LibreOffice Writer. It’s now up to an experienced
programmer to delve into this and hopefully fix it.

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